Codegen is disabled when scanning text tables with decimal columns. The message is "Decimal not yet supported for codegen."
The supported types use cross-compiled conversion functions in TextConverter::CodegenWriteSlot():
Decimal is a bit different to these functions because it accepts precision and scale parameters and the parsing functions are templated. However I think in principle the same approach can be used as for the other types.
Note: StringToDecimal() is a huge inline function and we risk run into codegen-time problems if we're scanning many decimal columns. We should experiment to see if this is a problem and if so try to mitigate it, e.g. by not moving the function out of the header.